The exotic quantum mechanical effects and electronic properties of quantum materials, associated with its strong electron-electron correlation, can be effectively manipulated through strain engineering by altering the atomic spacing and crystal symmetry. The present article illustrates the influence of mechanical strain on the electronic structure and spin texture of topological insulator Bi2Se3 using first principles density functional theory. The calculations reveal strain-induced shape variation of valence band maximum (VBM) and conduction band minimum (CBM) at Gamma-point, change of orbital dispersion along Gamma - M - K path, tuning of orbital occupancy, tweaking of bandgap, control of spin-orbit coupling (SOC) strength, and rotation of spin. Our result demonstrates the transition from indirect bandgap to direct bandgap of Bi2Se3 with strain and SOC, which occurs because of band crossing between the states mj = +/- 1/2 and +/- 3/2 of Se1 atoms. Strain with SOC controls the orientation of the spin, forms a wave-like pattern of the tip of spin vectors around the Fermi surface, and the spins originating from consecutive sides of the hexagon have opposite spin orientation at 1.43 % uniaxial strain. The results establish strain as an important parameter to independently control the electronic structures and spin texture of three-dimensional topological insulators and offers a promising material platform for designing topological electronic devices to accelerate the advances in the spintronic applications.

Photocatalytic water splitting offers a viable pathway for sustainable hydrogen production. In this study, first-principles density functional theory calculations were performed to explore the strain-dependent photocatalytic behaviour of a two-dimensional g-C3N4/GeS heterostructure. The heterostructure shows type-II band alignment with an indirect band gap of 2.17 eV, smaller than those of the individual g-C3N4 (2.81 eV) and GeS (3.31 eV) monolayers. Biaxial tensile strain up to 3

Photocatalytic water splitting has emerged as a promising strategy for sustainable hydrogen production using solar energy. In this work, we investigate the photocatalytic performance of the C2N monolayer under O2 and O3 exposure. Upon adsorption on C2N, both molecules form weakly interacting states, resulting in a metallic solution. However, after dissociation into atomic oxygen, the band gap of the oxidised monolayer increases relative to the pristine C2N, indicating a strong hybridisation of the C-O bond. These oxidised configurations exhibit band-edge positions that span the water redox potential, thereby reducing the likelihood of charge recombination and enhancing their separation. Interestingly, the optical absorption spectra show a blue shift relative to the pristine sample and lie in the visible region. These findings highlight the potential of oxygen- and ozone-modified C2N monolayers for applications in photocatalytic water splitting and sustainable hydrogen production.

Environmental oxidation has a great impact in tuning the physical, chemical and electronic properties of twodimensional (2D) materials, which can affect their practical applications in nanoscale engineering devices under ambient conditions. Aza-triphenylene is a recently synthesized 2D monolayer whose practical applications have not been systematically studied yet. In this study, we report for the first time, the adsorption and dissociation of O2 and H2O molecules on the surface of 2D aza-triphenylene monolayer through first principles calculations in combination with climbing image nudged elastic band (CINEB) method. The results indicate that both the O2 and H2O molecules weakly interact over the monolayer surface with an adsorption energy of-0.16 eV and-0.32 eV, respectively. In contrast, both the molecules exhibit resistance for dissociation due to the presence of energy barriers. The transition path indicates that molecular oxygen experiences two energy barriers (0.16 eV and 1.22 eV) before getting dissociated into atomic oxygen. However, the dissociation of H2O requires a larger energy barrier (2.3 eV and 0.86 eV) due to breaking of covalent bonds and transfer of hydrogen. The strong chemical adsorption of atomic oxygen and H+/OH-ions is due to the significant charge transfer from the monolayer to the adsorbate as evidenced from the charge density difference and Bader charge analysis. Moreover, the oxygen dissociated configuration exhibits a larger band gap as compared to the pristine azatriphenylene due to the strong hybridization between the p states of carbon and oxygen. our work predicts the robustness of aza-triphenylene monolayer against oxygen/water exposure thus ensuring their stability for device applications using these materials.

Light element based two dimensional (2D) materials are promising photocatalysts for hydrogen production via water splitting. Boron oxide (BO) is a recently synthesized 2D monolayer which has yet to be thoroughly explored for its potential applications. In this article, using first principles calculations, we report, for the first time, the visible-light photocatalytic activity of a BO monolayer for water splitting under mechanical strain and surface modification with single- and double-atom decorations (C, N, Si, Ge, P, As). The pristine BO monolayer exhibits an indirect band gap of 3.8 eV with band edges spanning the water redox potentials, but its optical absorption lies in the UV region ( 4.5 eV). Strain engineering tunes the band gap and band alignment with a minimal shifting in the optical absorption ( 0.5 eV). Single atom decoration produces a metallic state for elements like N, P, As, and an insulating state for single C, Si, Ge with a partial shifting in optical absorption. In contrast, double atom decoration produces substantial band gap reduction, improved band alignment, a pronounced red-shift in optical absorption into the visible range (1.6 to 3.2 eV) thus satisfying the criteria for water splitting. The stability of all the adsorbed configurations was confirmed by negative formation energy and ab-initio molecular dynamics simulations. These findings suggest BO monolayer functionalization can improve photocatalytic efficiency, providing hydrogen generation insights.

Understanding the interaction of various environmental oxidizing agents is important in determining the physical and chemical properties of 2D materials. Its impact holds great significance for the practical application of these materials in nanoscale devices functioning under ambient conditions. This study delves into the influence of O 2 and O 3 exposure on the structural and electronic characteristics of the C 2 N monolayer, focusing on the kinetics of adsorption and dissociation reactions. Employing first‐principles density‐functional theory calculations alongside climbing image nudged elastic band calculations, it is observed that the monolayer exhibits resistance to ozonation, evidenced by energy barriers of 0.56 eV. These processes are accompanied by the formation of COC groups. Furthermore, the dissociation mechanism involves charge transfers from the monolayer to the molecules. Notably, the dissociated configurations demonstrate higher bandgaps compared to the pristine monolayer, attributed to robust CO hybridization. These findings suggest the robustness of C 2 N monolayers against oxygen/ozone exposures, ensuring stability for devices incorporating these materials.

The development of renewable energy technologies is critical to addressing global challenges, such as climate change and ensuring energy sustainability. One promising approach is photocatalytic water splitting, which converts solar energy into hydrogen fuel. 2D materials, especially van der Waals heterostructures, have shown great potential for enhancing photocatalytic activity. In this study, we investigated the effects of vertical, uniaxial, and biaxial strain on the electronic band gap and band edge positions of the C2N/MoS2 van der Waals heterostructure through first-principles density functional theory using both Perdew-Burke-Ernzerhof (PBE) parameterization-based generalized gradient approximation (GGA) and strongly constrained and appropriately normed (SCAN) meta-GGA functional. Our results indicated that the SCAN functional provided more accurate results for band gap and band alignment, which are close to the experimental values as compared to the PBE functional. We discovered that the heterostructure exhibited a type-II band alignment, which is essential for efficient charge separation. Our calculation also established that the band edge positions straddled the water redox potential under compressive strain, suggesting its usefulness as an efficient photocatalyst to execute hydrogen/oxygen evolution reactions, whereas, for tensile uniaxial and biaxial strains, the band alignment is within the water oxidation/reduction potential. Furthermore, our study established that the meta-GGA SCAN functional yielded results similar to those of computationally expensive hybrid HSE functionals, reducing the computational cost of electronic structure calculations. Our findings provide valuable insights for designing 2D heterostructure devices with improved photocatalytic water-splitting performance.

The process of environmental oxidation is pivotal in determining the physical and chemical properties of two-dimensional (2D) materials. Its impact holds great significance for the practical application of these materials in nanoscale devices functioning under ambient conditions. This study delves into the influence of O2 and O3 exposure on the structural and electronic characteristics of the C2N monolayer, focusing on the kinetics of adsorption and dissociation reactions. Employing first-principles density functional theory calculations alongside climbing image nudged elastic band calculations, we observe that the C2N monolayer exhibits resistance to oxidation and ozonation, evidenced by energy barriers of 0.05 eV and 0.56 eV, respectively. These processes are accompanied by the formation of epoxide (C-O-C) groups. Furthermore, the dissociation mechanism involves charge transfers from the monolayer to the molecules. Notably, the dissociated configurations demonstrate higher bandgaps compared to the pristine C2N monolayer, attributed to robust C-O hybridization. These findings suggest the robustness of C2N monolayers against oxygen/ozone exposures, ensuring stability for devices incorporating these materials.

Lattice thermal conductivity ( κ L ) of the hexagon‐shaped nanocrystals cluster of Bi 2 Se 3 , prepared by the hot‐injection technique using nontoxic solvents, is studied. From the temperature‐dependent Raman spectra of Bi 2 Se 3 nanocrystals, the average Debye temperature ( θ D ) and Gruneisen parameter ( γ ) are calculated by adopting the bond‐order–length–strength correlation theory. The average room temperature κ L of Bi 2 Se 3 nanocrystals evaluated from the Slack model using θ D and γ is ≈1.1 Wm −1 K −1 . The κ L of Bi 2 Se 3 nanocrystals is larger than out‐of‐plane κ L (≈0.4 Wm −1 K −1 ) but close to the in‐plane κ L (≈1.4 Wm −1 K −1 ) simulated using the Boltzmann transport equation for phonon with three‐phonon scatterings. Nanostructuring introduces grain boundaries in the Bi 2 Se 3 that block the long mean free path of phonons physically, reduces the phonon mean free path, and decreases the κ L . The anisotropic phonon scattering introduced by the weak van der Waals force between adjacent quintuple layers in the out‐of‐plane direction, in addition to the acoustic–optical phonon scattering and anharmonicity, hinders the efficient transport of thermal energy in the Bi 2 Se 3 and results in a lower κ L . By utilizing materials with anisotropic thermal conductivity, thermoelectric devices can be designed to preferentially conduct heat in specific directions while minimizing heat loss in others.

Ion implantation is one of the versatile techniques for controllable doping of desired ions in solid materials. This work discusses the tunability of the structural, optical, and magnetic properties of hydrothermally synthesized rutile TiO2 microflowers composed of nanorods due to C ion implantations. The C ions of 1.5 MeV are implanted in the fluence range of 1 x 10(15) - 2 x 10(16) ions/cm(2) in normal incidence. The increase in the peak broadening of the X-ray diffraction (XRD) and Raman spectra in the ion-implanted TiO2 compared to the pristine TiO2 indicates the degradation of crystallinity due to the creation of lattice disorder and defects. The defect states are mainly attributed to the oxygen vacancy that assists in narrowing the optical band gap and causes magnetism. As the fluence increases, more defect states are created, which reduces the optical band gap from 3.04 to 2.98 eV and increases the Urbach energy from 144 to 193 meV. The ferromagnetic ordering with a tunable coercive field in the C-irradiated TiO2 is further evidenced through density functional theory (DFT) calculations, which indicate an interaction between the 3d states of Ti and 2p states of the O and C atoms. The spin-polarized total density of states for pristine TiO2 with Ti and O vacancies are calculated for the net magnetic moments to match the experimental results. The demonstration of C ion-implantation defect states plays an important role in tuning the phononic, photonic, and magnetic properties of TiO2 nanostructures, which are suitable for applications in versatile spintronic and optoelectronic devices based on nanorods.

Lattice thermal conductivity (κL) calculations using the Wiedemann-Franz law involve electrical conductivity, which introduces an error in the actual value of κL. We have adopted a non-contact measurement technique and calculated the κL from the temperature and power-dependent Raman spectra of the Bi2Se3 nanocrystals with truncated hexagon plate morphology stabilized in a hexagonal crystal structure. The hexagon plates of Bi2Se3 are 37 to 55 nm thick with lateral dimensions around 550 nm. These Bi2Se3 nanocrystals show three Raman lines, which agree with the theoretical prediction of A11g, E2g and A21g modes. Although the first-order thermal coefficient (-0.016) of Bi2Se3 nanocrystals is quite low, the room temperature κL ∼1.72 W m-1 K-1 is close to the value obtained from the simulation adopting a three-phonon process. The phonon lifetime of Bi2Se3 nanocrystals observed between ∼0.2 ps and 2 ps confirmed carrier-carrier thermalization with a small contribution from electron-electron and intraband electron-longitudinal-optical-phonon relaxation. The variations of phonon lifetime, Gruneisen parameter and κL of the mode frequencies outline the crucial role of the anharmonicity and acoustic-optical phonon scattering in reducing the κL of Bi2Se3. The non-contact measurements and relevant thermal property parameters open up exciting opportunities to address the anharmonic effects in other thermoelectric materials for obtaining a high figure of merit.

The improved photocatalytic water splitting using 2D materials has technological importance for economically viable renewable energy. The present study focuses on the effect of uniaxial, biaxial, and vertical strain on the energy gap and band edge positions of C2N/MoS2 van der Waals heterostructures through first-principles density functional theory using PBE and SCAN functionals. The calculations establish that SCAN functional provides comparatively much better results as compared to the PBE for the band gap and band alignment study. The heterostructure exhibits a type- II band alignment which is beneficial for the efficient separation of charge carriers. For a good photocatalyst, the band edge positions should straddle the water redox potentials. It is observed that for both compressive and tensile vertical strain, the water redox potential values lie within the valence band maximum (VBM) and conduction band minimum (CBM) of the heterostructure. On the other hand, for uniaxial and biaxial strain, the system can be used as a useful photocatalyst only for larger compressive strain, whereas for tensile strain, the energy gap between VBM and CBM keeps on decreasing and lie within the water oxidation/reduction potential. Our study also establishes that the meta-GGA SCAN functional shows similar results as compared to the computationally expensive hybrid HSE functionals. The present work can be extremely useful for experimentalists to design artificial heterostructure devices for better performance in photocatalytic water splitting.

Breaking time-reversal symmetry through magnetic impurities has been a useful strategy for achieving exotic physical phenomena in topological insulators for a long time. In contrast, reports on achieving ferromagnetism in topological insulators by introducing non-magnetic elements are very scarce. The present article illustrates Au ion implantation as an alternative tool to study the structural and magnetic properties in the Bi2Se3 nanocrystals grown by the chemical vapor deposition. The as-grown pristine Bi2Se3 exhibits triangular and hexagonal plate-like morphology, which undergoes structural distortion with increased Au ions. The room temperature Raman spectra show defect-induced phonon softening with ion fluence. The non-magnetic Bi2Se3 exhibit room temperature ferromagnetic hysteresis that originates from Bi vacancies which is verified from the density functional theory (DFT)calculations. The incorporation of Au ions initially decreases the magnetic moment up to the fluence 1 x 10(14) ions cm(-2) and enhances at higher fluences due to the formation of Au nanoclusters. The experimental and DFT results suggest a competition between the Se "4p" states and Au "5d" states for the non-monotonic behavior of magnetic properties. The present work on Au intercalation in Bi2Se3 can pave the way to design artificial topological insulator devices for potential technological applications for future spintronics and quantum computations.

The (0001) surface band structure evolution of Bi2Se3 is studied for different broken symmetries in the top quintuple layer (QL) of 8QLs slab. Each surface electronic structure shows dangling bond states (DBS) irrespective of broken symmetries introduced by the surface termination or layer vacancy. The DBS become topological (TDBS) in the presence of spin–orbit coupling (SOC) effect, which form multiple Dirac cones (DC) at the Γ and M points in the conduction band due to Bi1 termination in top QL. For Bi1 and Se2 terminations, the contribution of p‐orbital of Bi2 atom to DBS in the gap is negligible. The DBS eliminates the topological state of Bi2Se3 for all broken symmetries except Se2 layer vacancy. The band structure with only Se2 and Se3 layer vacancies shows a narrow‐bandgap semiconducting nature with a small bandgap value of 0.18 eV and 0.08 eV, respectively. The DC feature is observed in vacant atomic layer configurations with SOC. The layer vacancies created by Bi atoms form a metallic state, whereas those produced by Se atoms form a narrow‐bandgap semiconducting state. The introduction of broken symmetries in Bi2Se3 can be utilized in the tuning of surface states and TDBS for possible technological applications.

The band gap (E-g) engineering and Dirac point tuning of the (0001) surface of 8 QLs (quintuple layers) thick Bi2Se3 slab are explored using the first-principles density functional theory calculations by varying the strain. The strain on the Bi2Se3 slab primarily varies the bandwidth, modifies the p(z) - orbital population of Bi and moves the Dirac point of the (0001) surface of Bi2Se3. The Dirac cone feature of the (0001) surface of Bi2Se3 is preserved for the entire range of the biaxial strain. However, around 5% tensile uniaxial strain and even lower value of volume conservation strain annihilate the Dirac cone, which causes the loss of topological (0001) surface states of Bi2Se3. The biaxial strain provides ease in achieving the Dirac cone at the Fermi energy (E-F) than the uniaxial and volume conservation strains. Interestingly, the transition from direct E-g to indirect E-g state of the (0001) surface of Bi2Se3 is observed in the volume conservation strain-dependent E-g. The strain on Bi2Se3, significantly modifies the conduction band of Se2 atoms near E-F compared to Bi and Se1, and plays a vital role in the conduction of the (0001) surface of Bi2Se3. The atomic cohesive energy of the Bi2Se3 slab is very close to that of (0001) oriented nanocrystals extracted from the Raman spectra. The strain-dependent cohesive energy indicates that at a higher value of strain, the uniaxial and volume conservation strain provides better stability than that of the biaxial strain (0001) oriented growth of the Bi2Se3 nanocrystals. Our study establishes the relationship between the strained lattice and electronic structures of Bi2Se3, and more generally demonstrates the tuning of the Dirac point with the mechanical strain.

The (0001)-oriented hexagon-shaped nanocrystal plates of Bi2Se3 are synthesized by the hot-injection method using a nontoxic solvent. The Bi2Se3 hexagonal plates have a thickness of around 40 nm with a lateral dimension of 600 nm. The selected-area high-resolution transmission electron microscopy images show hexagonal lattice fringes with a lattice spacing of the (0001)-oriented hexagonal crystal structure. These nanocrystals exhibit a band transition energy of similar to 0.66 eV, which is 3 times that of Bi2Se3 single crystals. The first-principles density functional theory (DFT) calculations demonstrate that the expanded band gap (E-g) of these Bi2Se3 nanocrystals is due to the phenomenal surface band dispersion and the competition between covalent and van der Waals interactions that determines the topological state. The DFT calculations provide evidence for expansion of the (0001) surface E-g of Bi2Se3 from 0.3 to 0.63 eV with an increase of the surface for the same thickness. These Bi2Se3 nanocrystals dispersed between the Ag contact pads exhibit thermally activated behavior with Poole-Frenkel-type conduction due to the electron trapping/detrapping barriers between the nanocrystals. These results will further advance the understanding of fundamental charge-transport mechanisms in Bi2Se3, which can also be an essential parameter in the development of various electronic applications such as resistive memory switching and sensing devices.